Abstract

Cerebellar granule cell neurons are the most abundant neuron in the brain, and an important model for studying the evolution of neuronal population size. Allied to the relatively small number of prominent cell types, this makes the cerebellum an important model within which to study neurogenesis and its evolution in vertebrates. Previous in vitro and knockout studies have suggested that granule cell progenitor development in the external granule progenitor layer (EGL) of the developing cerebellum is regulated by both Notch and BMP signalling, with Notch being required to maintain progenitors in a proliferative state, and BMP to initiate differentiation. To test these specuations in vivo, we assayed activity of the TGF -β and Notch signalling using immunofluorescence and a Tol2 transposase-based reporter assay in the chick embryo. Interestingly, only a few cells in the inner external granule layer are seen to be responding to BMP signalling, whereas large numbers of granule cells in the inner granule layer are strongly responding to BMP . Notch signalling is prominent in, but not confined to, the more proliferative external granule layer, suggesting additional roles for Notch independent of granule progenitors within the developing cerebellum. Crucially, there appears to be a window of ‘signal-less’ initial external granule layer formation that may represent a relic of the events that first evolved in the amniote ancestor and facilitated the appearance of large, foliated cerebella.